HIGHLIGHTS
• MXenes are two-dimensional compounds that offer potentially attractive properties for use in electromobility applications.
• A perspective article now examines how MXenes can be used in a series of potential tribological applications involving electromobility.
• One intriguing approach is to use MXenes in combination with ionic liquids

MXenes were discovered over 10 years ago and have emerged as potential candidates for use in a variety of applications including energy storage, functional textiles and telecommunications. This family of compounds has been under evaluation due to their ability to tune a specific type to meet a particular application. 

This article has highlighted several studies done with MXenes that have pertinence to lubrication. As a review, MXenes are two-dimensional compounds that have the general formula M_n+1X_nT_x where M is an early transition metal, X represents either carbon and/or nitrogen and T are termination groups that may include oxygen, hydroxyl and/or fluorine.

MAX phases, which are early transition metal carbides, are the precursors to MXenes. The traditional process to synthesize MXenes is to etch the MAX phases using aqueous hydrofluoric acid. A second method has been developed where a MAX phase is etched with a molten salt under anhydrous conditions. The advantage of this approach is that the presence of water even in small concentrations can interfere in certain applications such as electromobility.

A previous TLT article¹ describes a non-aqueous synthesis route for producing MXenes from ammonium bifluoride and a polar solvent such as propylene carbonate. The researchers demonstrated the benefit of this approach by incorporating the MXenes produced as anodes into sodium-ion batteries. The battery capacity obtained was nearly double that was found when the same MXene was synthesized using aqueous hydrofluoric acid etching. 

The move toward electromobility presents a whole new set of challenges for lubricants and for the durability of materials. Andreas Rosenkranz, professor in the Department of Chemical Engineering, Biotechnology, and Materials at Universidad de Chile in Santiago, Chile says, “Electric vehicle powertrains operate in harsh operating conditions that involve higher bearing rotational speeds, and rapidly changing torques that can lead to frictional and wear issues among transmissions, batteries and electric motors. This environment creates new mechanical and electrical problems that have not been faced in internal combustion powered vehicles.”

Other two-dimensional materials such as graphene and hexagonal boron nitride (h-BN) have been under evaluation in electromobility applications, but limitations have been found in their ability to perform under the challenging electrical and mechanical environment of electric vehicles. Rosenkranz says, “These two-dimensional materials have displayed promise, but there have been negative results in electromobility applications due to issues with phase, structure and/or compositional stability over a long-term operating time.”

MXenes have not been widely evaluated in electromobility applications, as Rosenkranz reported that only two or three papers have been published. Rosenkranz and his colleagues, including STLE member Dr. Diana Berman, Dr. Brian Wyatt, Dr. Babak Anasori, Dr. Leonardo Farfan-Cabrera and STLE Past President Dr. Ali Erdemir have now published a perspective article2 on MXenes to examine how this family of materials may provide answers to a wide range of electromobility applications. 



Potential tribological applications
Rosenkranz and his colleagues state that MXenes are attractive candidates for use in electromobility because they offer a unique property combinations that provide good mechanical properties, outstanding electrical conductivity, tunable thermal conductivity and controllable chemical stability. He says, “MXenes are reactive species that can be used in a beneficial manner in electromobility applications to generate thin surface films that should display excellent wear resistance and durability. Adjusting MXenes’ thermal conductivity can promote heat dissipation, which is extremely important in applications such as electric vehicles.”

The authors listed a series of potential tribological applications where MXenes can provide beneficial contributions. MXenes have been evaluated as fluid additives in tribological applications. Past research has demonstrated the ability of a small treat rate of a titanium-based MXene to increase thermal conductivity and reduce the resulting viscosity in lubricants based on either silicon or polyalphaolefin (PAO) synthetic base stocks. An additional benefit is a reduction in friction across multiple lubrication regimes in both silicon and PAO. 

Electrical contacts, such as electric vehicle charging connectors, could be a potential application for MXene coatings due to their high electrical conductivity. MXenes are appealing because currently used materials such as silver and gold-based alloys have limitations including high cost, durability and formability issues. The authors point out that combining MXenes with other solid lubricants such as molybdenum disulfide could produce excellent friction and wear properties particularly in vacuum conditions. This may be useful particularly in electromobility systems designed for outer space.

An intriguing approach for working with MXenes is to combine them with ionic liquids. Rosenkranz says, “Ionic liquids have shown promise in reducing the friction and wear of sliding surfaces. Combining them with MXenes by tailoring a positively charged ionic liquid with a negatively charged MXenes could produce excellent tribofilms on metallic surfaces. Ionic liquids can furnish efficient ion transport and environmental stability. When combined with MXenes conductive, high-surface area scaffolding, the result could be hybrid electrolytes that may contribute improved ionic conductivity and stability to batteries over a wide temperature range.”

One negative property that needs to be addressed is MXenes’ susceptibility to oxidation-based corrosion particularly in humid or saline environments. Rosenkranz says, “There are different ways that this vulnerability can be addressed through the use of rich, surface chemistry that can be used to functionalize MXenes depending upon the application. One approach is to utilize titanium carbide-based MXenes which can undergo oxidation to form a protective titanium dioxide layer on the surface. A second option is to react hydroxyl end groups with silicon-based functionalities to form silanes. The result is a change in the surface environment. Another method is to encapsulate the MXene within a polymer to form a composite.”

The authors conclude by stating that the versatility of MXenes through the ability of developers to fine tune their properties should open up opportunities for them to contribute to future electromobility applications. Additional information can be found in the perspective article² or by contacting Rosenkranz at arosenkranz@ing.uchile.cl.

REFERENCES
1. Canter, N. (2020), “New approach for producing MXenes,” TLT, 76 (7), pp 14-15. Available at www.stle.org/files/TLTArchives/2020/07_July/Tech_Beat_III.aspx.
2. Berman, D., Wyatt, B., Anasori, B., Farfan-Cabrera, L, Erdemir, A. and Rosenkranz, A. (2025), “Potential of 2D MXenes in electromobility,” Materials Today, 90, pp. 724-739.